The invention is related to the field of free-space wireless optical communications, and more particularly, to improved optical telescopes for free-space wireless optical communication systems.
In wireless optical communication systems, the optical signal propagates in free space. In contrast to radio frequency (RF) communication systems, optical wireless communication systems are extremely directional. Thus, precise alignment is required between the transmitting unit and the receiving unit, often referred to as transmitting and receiving telescopes, respectively. The highly directional nature of wireless optical communication systems, however, provides the advantage of improved security, since the optical signal can only be intercepted along the path of the transmitted light. In addition, the optical portion of the spectrum is not regulated by the government. Thus, a government license is not required to operate the optical transmitter and receiver telescopes, unlike a comparable radio frequency (RF) wireless communication system. More importantly, the bandwidth or information carrying capacity of optical wireless systems is much greater than that of RF wireless communication systems.
Wireless optical communication systems have an advantage over fiber-based optical communication systems as well, since wireless communication systems do not require a physical connection between the transmitter and the receiver. In an urban environment, it can be difficult to install a physical connection, such as an optical fiber, between two buildings, especially if the buildings are separated by a street, another building or a body of water. A wireless optical link only requires an unobstructed path between the transmitter and the receiver, which is generally easier to achieve in an urban environment than a physical link.
FIG. 1 illustrates a conventional wireless optical system 100. As shown in FIG. 1, a conventional wireless optical system 100 typically includes a transmitting telescope 110, for forming a transmitted beam 115 that is aimed at a receiving telescope 120. Typically, the optical signal to be transmitted is originally emitted from a semiconductor laser and then may be amplified with an optical amplifier. The emitting facet of the laser (or an optical fiber into which the laser is coupled) lies at the front focal plane of the transmitting telescope 110. The received signal is typically collected with a photodetector (or an optical fiber connected to the photodetector) positioned at the focal plane of the receiving telescope 120. For a more detailed discussion of conventional wireless optical systems 100, see, for example, P. F. Szajowski, xe2x80x9cKey Elements of High-Speed WDM Terrestrial Free-Space Optical Communications Systems,xe2x80x9d SPIE Paper No. 3932-01, Photonics West (Jan. 2000), incorporated by reference herein.
In many wireless optical systems, it is desirable for a single transmitting telescope to communicate with a number of receiving telescopes (often referred to as xe2x80x9cpoint-to-multipointxe2x80x9d communications), or for a single receiving telescope to receive signals from a number of transmitting telescopes (xe2x80x9cmultipoint-to-pointxe2x80x9d communications). With conventional wireless optical systems, however, point-to-multipoint communications typically require a dedicated transmitting and receiving telescope for each optical path. See, for example, PCT application Ser. Nos. PCT/US99/14710 and PCT/US99/15973. A need therefore exists for a point-to-multipoint communication system that permits a single transmitting telescope to communicate with a number of distributed receiving telescopes.
Generally, a free-space wireless optical communication system is disclosed that satisfies the above-stated objectives of improved bandwidth, link range and reliability, relative to conventional designs. The disclosed free-space wireless optical communication system utilizes a telescope design having aspherical mirrors, such as a Ritchey-Chretien (RC) telescope. RC telescopes are characterized by a concave primary mirror and a convex secondary mirror each having a hyperbolic shape.
The present invention provides a mirror configuration that allows the primary and secondary mirrors to be positioned closer together than conventional designs, allowing for a very compact system. The disclosed mirrors are not formed of thick bulk glass. Thus, the mirrors can be thin and lightweight, allowing for a lightweight telescope unit. In addition, the present invention provides a larger focal plane that allows for automatic alignment between a transmitter and receiver with a stationary or fixed mirror design, further contributing to a lower fabrication cost. Furthermore, the mirrors can be fabricated with large diameters, to allow the transmission of high beam intensities at eye-safe levels since the energy is spread over a large area.
Among other benefits, the larger focal plane permits an nxc3x97n fiber array to be positioned in the focal plane of the RC optical telescope, thereby enabling point-to-multipoint communications with a single optical telescope. Each fiber in the nxc3x97n fiber array of a transmitting telescope can be focused on a different receiving telescope in a wireless optical communication system. In this manner, each fiber in the nxc3x97n fiber array sends optical energy over a distinct path of address a given receiving telescope. Likewise, for a multipoint-to-point communication system, an nxc3x97n fiber array can be positioned in the focal plane 330 of the RC optical receiving telescope, with each fiber in the nxc3x97n fiber array receiving optical energy over a distinct path from a given transmitting telescope.
A number of fabrication techniques are also disclosed that permit the optical telescopes of the present invention to be fabricated at a reasonable cost that permits such optical telescopes to be deployed in wireless optical communication systems.
A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings.